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GTS and US liquor laws

Please forgive me if this has been answered before -- I did a search and couldn't find anything. According to Regan and Regan's The Book of Bourbon, bourbon must finish its second distillation at no higher than 160 proof and must age at no higher than 125 proof in charred new oak barrels for not less than 2 years (p. 212). It's also my understanding that whiskey tends to become lower in proof as it ages because alcohol more readily evaporates through the barrel than does water. If that's the case, how can it be that GTS (and other bourbons) sell at proofs higher than 125? Not trying to cause trouble, just legitimately curious by an apparent conundrum.

Re: GTS and US liquor laws

Originally Posted by jcusey

It's also my understanding that whiskey tends to become lower in proof as it ages because alcohol more readily evaporates through the barrel than does water.

No trouble. The disconnect is in the statement above. In fact, depending on a variety of conditions, whiskey in the barrel can go up in proof, down in proof, or remain essentially unchanged. In the climates of Kentucky and Tennessee, especially in the hottest parts of the warehouses, the proof tends to go up and especially after long aging, barrel proofs above 140 are common.

Re: GTS and US liquor laws

Originally Posted by cowdery

No trouble. The disconnect is in the statement above. In fact, depending on a variety of conditions, whiskey in the barrel can go up in proof, down in proof, or remain essentially unchanged. In the climates of Kentucky and Tennessee, especially in the hottest parts of the warehouses, the proof tends to go up and especially after long aging, barrel proofs above 140 are common.

Thank you for your response, cowdery. So at higher temperatures, the water in the spirit is more volatile than the alcohol? What other conditions could cause this to happen?

Re: GTS and US liquor laws

I've never gotten a crystal clear explanation. I'm told that the factors are humidity, atmospheric pressure, the size of water vapor molecules, the size of alcohol vapor molecules, the density of the wood, the ratio of air to liquid inside the barrel, and who knows what else, sun spots maybe. It just happens that way. In Scotland, proof invariably goes down, in Kentucky it generally (but not always) goes up.